The present invention relates to variable gain mixers and amplifiers, particularly in direct conversion transceivers.
The present invention finds application in many contexts. Direct conversion transceivers, in which conversion between a radio frequency signal and a modulation or demodulation frequency is done in one stage, are gaining increasing importance. Direct conversion transceivers draw less power than multistage receivers. Since the direct conversion receiver has low frequency output, AC coupling between stages is not viable. Consequently DC isolation of an amplifier or mixer stage is not possible.
It is highly desirable to be able to be able to change gain of an amplifier or mixer while keeping the common mode operating point constant. This is difficult in a non-isolated stage. Improved designs concentrate on drawing less power. Lower supply voltages, 2.5, 2.7 volts, are being utilized than in the past. With smaller supply voltages, smaller variations in common mode operating level represent a greater percentage variance. Also, fewer components can be stacked between power and ground.
It is therefore an object of the present invention to provide a mixer/amplifier capable of providing variable gain while maintaining a substantially constant common mode operating voltage level. It is a further object of the present invention to maintain a substantially constant operating voltage in new, low voltage designs. It is a more specific object, in one form, to provide a mixer/amplifier in low-power direct conversion receiver.
Briefly stated, in accordance with the present invention, a low noise amplifier is provided in which a gain control signal is provided through a differential current source. Two output currents I1 and I2 are provided. I1+I2=a constant. I1 and I2 are mirrored in first and second paths, one including amplifier transistors and optionally a Gilbert cell multiplier. Irrespective of amplifier gain, the current through the two paths remains equal to I1 and I2, and common mode voltage remains constant.